According to the results of the census of 2015 more than half of the South Korean population (56.1%) declared themselves not affiliated with any religious organizations. In a 2012 survey, 52% declared themselves "religious", 31% said they were "not religious" and 15% identified themselves as "convinced atheists". Of the people who are affiliated with a religious organization, most are Christians and Buddhists. 

Christianity is South Korea's largest organized religion, accounting for more than half of all South Korean adherents of religious organizations. There are approximately 13.5 million Christians in South Korea today; about two thirds of them belonging to Protestant churches, and the rest to the Catholic Church.

Among Christian denominations, Presbyterianism is the largest. About nine million people belong to one of the hundred different Presbyterian churches; the biggest ones are the HapDong Presbyterian Church, TongHap Presbyterian Church, the Koshin Presbyterian Church. South Korea is also the second-largest missionary-sending nation, after the United States.

Buddhism was introduced to Korea in the 4th century. It became soon a dominant religion in the southeastern kingdom of Silla, the region that hitherto hosts the strongest concentration of Buddhists in South Korea. In the other states of the Three Kingdoms Period, Goguryeo and Baekje, it was made the state religion respectively in 372 and 528. It remained the state religion in Later Silla (North South States Period) and Goryeo. Today, South Korea has about 7 million Buddhists, most of them affiliated to the Jogye Order. Most of the National Treasures of South Korea are Buddhist artifacts.